Alzheimer's disease risk linked to health of extended family members

It's not just parents' and grandparents' histories you have to worry about

Having a great-grandparent or even a cousin — referred to as “third-degree relatives” — with Alzheimer's disease may raise your own risk of developing the disease, according to new research.

It’s already known that children of Alzheimer’s patients are at higher risk for the disease. But in the new study, people with the progressive neurological disease in their extended family were also at a higher risk compared to those with no family history, according to University of Utah School of Medicine researchers, as reported in the journal Neurology.

First-degree relatives include parents and siblings who share both parents. Second-degree relatives include grandparents, blood-related aunts and uncles, and siblings who share one parent. Third-degree relatives include great-grandparents, great uncles, great aunts and first cousins.

“The big picture message is that this reiterates how important and powerful (both close and distant) family history can be for risk prediction,” said the study’s lead author, Lisa Cannon-Albright, told Reuters.

The new study was possible only because researchers were able to tap into a very complete genealogy via the Utah Population Database, which includes information on families dating back to the original pioneers who settled the state in the 1800s. The family histories are linked to Utah death certificates, which show not only a cause of death, but also other contributing causes in the majority of cases, Reuters reports.

In that database, researchers analyzed data from more than 270,000 people who had at least three generations of genealogy connected to the original Utah pioneers including genealogy data for both parents, all four grandparents and at least six of eight great-grandparents. Of those, 4,436 have a death certificate that indicates Alzheimer's disease as a cause of death, according to a press release on the study.

Researchers found that people with one first-degree relative with Alzheimer's had 73 percent increased risk of developing the disease. A diagnosis was four times more likely for people with two first-degree relatives who had the disease; two-and-half more times likely with three; and nearly 15 times more likely with four.

Of the 21 people in the study with four first-degree relatives with Alzheimer's, six had the disease; researchers would have expected only 0.4 people to develop the disease, according to the release.

Those who had only third-degree relatives, and three such relatives, with Alzheimer's disease had a 43 percent greater risk of developing the disease. An example of this would be two great-grandparents with the disease, along with one great uncle, but no parents or grandparents with the disease.

One might feel "safe" having no first- or second-degree relatives with Alzheimer’s, the researchers found that people with a history of Alzheimer’s in three or more third-degree relatives — great-grandparents, great-uncles and -aunts and first cousins — were 1.43 times as likely as those with no family history to develop the disease, Reuters reports.

Limitations of the study include that not all individuals dying from Alzheimer's disease may have had a death certificate listing it as cause of death. In fact, the researchers note that death certificates are known to underreport the prevalence of the disease.

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